Light quenching processes that rely on the interaction of two dyes as their spatial relationship changes can be used in convenient processes for detecting and/or identifying nucleotide sequences and other biological phenomena. In one such method the change in fluorescence of a fluorescent donor or quencher can be monitored as two oligonucleotides (one containing a donor and one containing a quencher) bind to each other through hybridization. The binding can be detected without intervening purification steps that separate unhybridized from hybridized oligonucleotides.
Another method for detecting hybridization using fluorophores and quenchers is to link fluorescent donors and quenchers to a single oligonucleotide such that there is a detectable difference in fluorescence when the oligonucleotide is unhybridized as compared to when it is hybridized to its complementary sequence. For example, a partially self-complementary oligonucleotide designed to form a hairpin can be labeled with a fluorescent donor at one end and a quencher at the other end. Intramolecular annealing into the hairpin form can bring the donor and quencher into sufficient proximity for fluorescent quenching to occur. Intermolecular annealing of such an oligonucleotide to a target sequence disrupts the hairpin, which increases the distance between the donor and quencher and results in an increase in the fluorescent signal of the donor.
However, oligonucleotides are not required to have hairpins for this later method to work efficiently. The fluorophore and quencher can be placed on an oligonucleotide such that when it is unhybridized and in a random coil conformation, the quencher is able to quench fluorescence from the fluorophore. Once the oligonucleotide hybridizes to a complementary nucleotide sequence it becomes more extended and the distance between the fluorophore and quencher is increased, resulting in increased fluorescence.
Oligonucleotides labeled in a similar manner can also be used to monitor the kinetics of PCR amplification. In one version of this method the oligonucleotides are designed to hybridize to the 3′ side (“downstream”) of an amplification primer so that the 5′-3′ exonuclease activity of a polymerase digests the 5′ end of the probe, cleaving off one of the dyes. The fluorescence intensity of the sample increases and can be monitored as the probe is digested during the course of amplification.
Similar oligonucleotide compositions find use in other molecular/cellular biology and diagnostic assays, such as in end-point PCR, in situ hybridizations, in vivo DNA and RNA species detection, single nucleotide polymorphism (SNPs) analysis, enzyme assays, and in vivo and in vitro whole cell assays.
Perhaps the most common mechanism of fluorescent quenching is known as FRET (fluorescent resonance energy transfer). For FRET to occur a fluorescent donor and a fluorescent quencher must be within a suitable distance for the quencher to absorb energy from the donor. In addition, there must be overlap between the emission spectrum of the fluorescent donor and the absorbance spectrum of the quencher. This requirement complicates the design of probes that utilize FRET because not all potential quencher/donor pairs can be used. For example, the quencher known as BHQ-1, which absorbs light in the wavelength range of about 520-550 nm, can quench the fluorescent light emitted from the fluorophore, fluorescein, which fluoresces maximally at about 520 nm. In contrast, the quencher BHQ-3, which absorbs light in the wavelength range of about 650-700 nm would be almost completely ineffective at quenching the fluorescence of fluorescein through FRET but would be quite effective at quenching the fluorescence of the fluorophore known as Cy5 which fluoresces at about 670 nm. In general, the number of quenchers known that are capable of quenching the fluorescence of any given fluorophore is quite limited. For example with fluorescein, only a limited number of suitable quenchers are known and they are quite expensive to purchase commercially. Because fluorescein is one of the most commonly used fluorophores, new quenchers that can quench fluorescent light in the 520 nm range of fluorescein are needed. Similarly, quenchers for other known fluorophores are also needed.
Ideally, new quenchers will not fluoresce so that background fluorescence is minimized. This will allow for an increased signal to noise ratio in the probes that contain them, resulting in more sensitive probes. In addition, the lack of a secondary fluorescence facilitates the use of additional fluorophores in multiplexed assay formats which utilize multiple distinct probes each containing a different fluorophore. If a quencher emitted light in a region, then additional probes could not bear fluorophores that emit light in that region.
New quenchers should also have physical properties that facilitate their purification and the purification of probes into which they are incorporated. They should also be chemically stable so that they can be incorporated into biological probes and used in assays without significant degradation. The quenchers should contain suitable reactive moieties to provide for their convenient incorporation into biologically relevant compounds such as lipids, nucleic acids, polypeptides, and more specifically antigens, steroids, vitamins, drugs, haptens, metabolites, toxins, environmental pollutants, amino acids, peptides, proteins, nucleotides, oligonucleotides, polynucleotides, carbohydrates, and the like. Lastly, the most useful compositions should be easily manufactured.
The invention provides nonfluorescing, fluorescence-quenching compositions, some of which have strong fluorescence quenching properties in the 520 nm range. Moreover, the quenchers of the present invention are chemically stable and can be easily manufactured and purified. The compositions can be incorporated into biologically relevant compounds and, in many cases, impart useful purification properties to these compounds. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.